Frequency control circuit



July 3, 1945. M. G. CROSBY FREQUENCY CONTROL CIRCUIT Filed Jan. 27, 1943 2 Sheets-Sheet 1 INVENTOR Ala/PRAY l. (/Posv;

ATTORNEY -July 3, 1945. M. G. CROSBY 2,379,689

FREQUENCY CONTROL CIRCUIT UFF ENE ATTORNEY Patented 'July 3, 1945 FREQUENCY CONTROL CIRCUIT Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of of Delaware America, a corporation Application January 27, 1943, Serial No. 473,681

(Cl. Z50-36) iClaim.

My present invention relates to frequency control circuits, and more particularly to automatic frequency control (AFC) circuits of the type responsive to shifts in carrier frequency from a predetermined frequency value.

One of the main objects of my present invention is to improve the gain of, and simplify, automatic frequency control systems of the type disclosed in my copending application Serial No. 393,339, filed May 14, 1941.

Another important object of this invention is the provision of a frequency discrimlnator circuit upon which is applied modulated high frequency energy of a predetermined mean frequency, there being provided a two phase motor device for maintaining the magnitude of the mean frequency of the modulated energy, a low frequency motor-energizing current source being provided to feed current to said discriminator and said motor device in quadrature phase relation, and said motor device being energized as a function of the extent and sense of frequency shift of the modulated energy from said mean frequency by virtue of the low frequency current appearing at the discriminator output.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claim; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description, taken in connection with the drawings, in which I have indicated diagrammatically a circuit organization whereby my invention'may be carried into effect.

Fig. l shows a system of AFC employing the invention.

Fig. 2 illustrates the rectifier outputs in the -in-tune state,

Fig. 3 shows the rectifier outputs for off-tune state, and Fig. 3a shows the resultingv current flow through coil 2l,

Figs. 4 and 4a show similar relations to Figs. 3 and 3a respectively for oil-tune state in the opposite sense.

Referring, now, to the accompanying drawings, I have shown my present invention applied generally to an AFC system of a well known form. Genetically, there are shown an oscillator I having a resonant tank circuit 2. The latter has a tuning, or frequency selecting, instrumentality 3. The oscillator, which may be considered as operative in any desired frequency band, feeds its oscillations to a network 4 indicated as a "Modulator." The latter is to be construed as generic to a transmitter modulator or a superheterodyne receiver converter. The input electrode of the modulator will he connected to a source of signals. The modulator output energy, in the form of signal-modulated oscillations whose mean frequency is that of the frequency of tank circuit 2, is applied to a selective amplifier having input and output circuits each tuned to the aforesaid mean frequency. The numeral l designates the tuned output circuit of amplifier 5.

The numeral 1 denotes a transmission path feeding a utilizing circuit with the output energy of the selective amplifier 5. Where the system is a superheterodyne receiver, the oscillator l will be the usual local oscillator which is tunable, by tuning condenser I, over a range of frequencies. The moduator l will, therefore, be the conventional first detector whose tunable input circuit is adjustable over a range of carrier frequencies. 'I'he modulator output circuit will, accordingly, be flxedly resonated to the intermediate frequency (I. F.) value. Amplifier 5 will be tuned to the operating I. F. value, as will the resonant output circuit 6. The path 1 may then feed the I. F. energy to additional I. F. ampliers and to a second detector followed by audio frequency amplifiers and audio reproducer.

If the system is a transmitter, then the modulator l would be fed with audio signals, and with carrier oscillations of a predetermined frequency from a master oscillator l. The output energy of modulator l will be at the frequency of the oscillator I, but modulated in accordance with the signals fed to modulator 4. The amplifier 5 will then feed the modulated oscillatory energy to a radiating system over path 1.

It will now be seen that in either system there is developed at circuit B modulated canier frequency energy. The carrier frequency may be in the kilocycle (kc.) range. or in the megacycle (mc.) range. As is well known in present-day super-heterodyne receivers, a change in oscillator frequency is accompanied concurrently by a change in the signal frequency of the converter input circuit. The resulting I. F. signal output energy is not accurately tuned in unless its mean frequency is equal to the resonant frequency of circuit 8. An AFC circuit responds to a frequency difference between the I. F. signal energy and the predetermined I. F. value to vary the oscillator tuning ina sense, and to an extent, such as to cause the frequency difference to be reduced. Similarly, in the case of a transmitter, a change in constants of the oscillator i may create a frequency difference to exist between the predetermined frequency value of amplifier 5 and the mean frequency of the modulated carrier energy output of the amplifier.

A well known' form of discriminator-rectiner network is shown coupled to circuit I. The function of the network is to provide a control voltage, for APC purposes, in response to a predetermined frequency shift of the carrier of the modulated wave energy applied to circuit I. The specific form of circuit is that disclosed, and claimed. by B. W. Seeley in his U. S. Patent No. 2,121,103, granted June 2l, 1938. Accordingly, the action of the discriminator-rectiiler will not be described in detail, reference being made to that patent. It will be suiiicient for the purposes cf this application to explain that a resonant circuit I, tuned to the same frequencyas circuit I, is magnetically coupled thereto. A direct current blocking condenser I is connected directly from the high potential side of circuit I to the midpoint or the coil of circuit I.

The diode rectiilers II and II, which may be provided by a 6K8 type tube, are arranged in polarity opposition. 'I'he anodes of the diodes are connected to respectively opposite sides of common input circuit I. The cathodes are connected by a center-tapped resistor which is thereby divided into series load resistors I2 and I2'. Each of load resistors I2 and I2' is bypassed for carrier frequency currents by a respective shunt condenser. 'I'he cathode end of resistor I 2' is grounded. The junction of load resistors I2 and Il' is connected to the midpoint of the cnil of circuit I through a path consisting of coils II and II in series. Coil II is the secondary of a power input transformer II. The primary winding II of the transformer is connected across a source of low frequency current. For example, the latter may be the usual 60 cycle line current employed to energize the various tubes of the system. The invention is not limited to the use of 60 cycle current, and it is to be clearly understood that any other value of low frequency may be used in place of the specific 60 cycle value.

The cathode end of resistor I2 is connected to the control grid of tube II, shown as a tricde merely by way of illustration, by a direct current blocking condenser I1'. The cathode of tube I1 may be connected to ground by the bias resistorV II, and the control grid returned to ground by the resistor II. A further amplifier. as for example a pentode, is designated by numeral 2l. The bias resistor is II. and the input grid of tube II is coupled by direct current blocking condenser 22 to the plate end oi' the plate load resistor 2l. 'l'he screen and plate of tube II may be connected to the lower end of resistor 23, the winding 24 of the two phase motor device being included in the plate circuit. The condenser 2| shunts winding Il, and tunes the winding to resonance with the I0 cycle current.

The numeral II designates the roter of the two phase motor. The latter is schematically represented, since its construction is well known in the electrical art. The energizing windings of the motor rotor II are represented by numerals 2l and 2l. These windings are wound on a common iron core, and will each carry 60 cycle current derived from the 60 cycle source. The winding II has one end thereof connected through phase shifter condenser Il to one end of primary 'lo winding II of transformer II. The opposite end of coil 25 is connected. to the opposite end oi' 4winding II. In other words, the coil 2l is arranged in a closed series circuit with the primary winding II and Phascshifterli.

The rotatableelement of the two phase motor is indicated as being arranged for mechanical coupling with the adjustable, or rotor. element of variable condenser I. 'I'he numeral I! indicates a dotted line which is to be understood as schematically representing any desired type of mechanicaI coupling between the rotatable element of the two phase motor and the adjustable element of the tuning mechanism of oscillator I. 1The motor need not adJust the rotor of the tuning condenser I, since it can operate an auxiliary Dadder condenser arranged in shunt with condenser 3. This use of an auxiliary adjusting condenser will be desirable in most cases, and especially where frequency control is desired for small ranges on either side of the operating oscillator frequency.

In explaining the operation of the system which has been described, it is first pointed out that cycle current will iiow through windings 2l and 2l. Furthermore, these currents must bein phase quadrature in order to energize the rotor II. The currents through the windings 24 and 2l have been indicated by arrows designated respectively as fw and l'n. The 60 cycle current flowing through winding 2l is derived directly from the discriminator-rectiiler network by virtue of the 60 cycle current injected into the common input circuit I. The 60 cycle current flowing through winding 2l is derived directly from the 60 cycle source, but the phase shifter condenser Il creates a degree phase shift in the 60 cycle current flowing to the winding 2l. Hence. in order to energize the motor it is merely necessary for 60 cycle current to appear in the winding 2l.

However, the 60 cycle current will not appear in winding 2l unless the modulated carrier energy appearing at the circuit I has a mean frequency which is not equal to the predetermined frequency of circuits I and I. In other words, for modulated canier energy whose mean frequency is equal to the predetermined frequency of input circuit I the discriminator-rectifier network has no output with a 60 cycle current oom- 4l.' Ponent. However, if the modulated carrier energy developed across circuit I has its mean frequency shifted from the aforementioned predetermined frequency of circuits I and I, that is Ifor otr-tune condition, then a 60 cycle voltage component will appear across load II-II' and be transmitted through tubes II and II thereby to iiow through winding II.

The tubes I1 and ZI function as 60 cycle current ampliilers. When the mean frequency of Il the applied modulated carrier energy is equal to the predetermined frequency of each of circuits I-I, the radio frequency voltages applied to the rectifiers II and II will be of equal magnitude. Ihis arises in virtue of the construction of the discriminator network, and has been explained in detail in the aforesaid Seeley patent. The equal radio frequency voltages applied to rectiiiers II and Il are rectified into direct current voltages developed across each of resistors I2 and I2'. These direct current voltages are of equal magnitude. Hence, both diodes are biased equally, and, since both are fed equal amounts of the 60 cycle voltage from II, the dierential output of 60 cycles appearing across I2 and I2 will be zero. The diode output resistors are connected in phase opposition. Consequently. there will be zero 60 cycle voltage in this case. This is equivaient to showing that there will be zero 60 cycle current flow through winding 2l.

0n the other hand. if the mean frequency of the applied modulated carrier energy departs from the predetermined frequency in one direction, one of the rectlilers will receive more radio frequency voltage than the other. This places unequal biases on the diodes so that their output of 60 cycle component will be unequal. The dif ferential output will then no longer cancel so that 60 cycle voltage will be developed at the cathode end of resistor I2, and the magnitude and polar ity of the voltage will depend upon the extent. and direction of shift of the aforesaid mean frequency. Hence, 60 cycle current flow through winding 24 will have a. magnitude and direction dependent upon the extent and direction of shift of the mean frequency of the applied modulated carrier energy from the predetermined frequency of circuits 6 and 8. In this way, there is provided a method of operating the two-phase motor at variable speed and in alternative directions. This permits an accurate control over adjustments of the tuning instrumentality of circuit 2 of oscillator I.

In Figs. 2, 3, 3a, 4 and 4a there is graphically presented the functioning of the circuit embodying my invention. In Fig. 2 there is shown the in-tune" voltage outputs of rectifiers i and Il so far as 60 cycle current is concerned. Since both diodes are equally biased, each acts as a half wave rectifier. The outputs are equal and cancel. Fig. 3 shows the off-tune" state when the mean frequency of applied carrier energy has shifted to one side of the predetermined reference frequency. The outputof rectifier l0 exceeds that of rectifier Il, and the resultant output is shown in dotted curves in Fig. 3. Fig. 3a shows the resulting voltage in full line curve, and the dotted curve in that figure shows the effect of tuned circuit 24--24 in removing harmonics. The 60 cycle current through winding 24 has been reduced ln intensity.

Figs. 4 and 4a show the manner in which the phase of the cycle current is changed when the "off-tune is in the opposite sense. Figs. 4 and 4a correspond respectively to Figs. 3 and 3a.

While I have indicated and described one system for carrying my invention into effect, it will be apparent to one skilled in the art that my in vention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claim.

What I claim is:

In combination with a high frequency oscillator having means for adjusting the frequency thereof, a two phase motor provided with a pair of energizing windings, a source of alternating current, means for applying current from said source to one of said windings, a discriminatorrectifier network comprising a pair of diodes each having an anode and cathode, an input circuit coupled between the anodes of said diodes, a resistor connected between the cathodes of the diodes, a second input circuit connected between a point on the first input circuit and a point on said resistor, means for impressing oscillatory energy derived from said oscillator upon the first input circuit whereby said diodes develop across said resistor rectified voltages which bias the diodes equally in response to the frequency of the oscillatory energy being equal to a predetermined reference frequency, means to apply current from the alternating current source to said second input circuit, and connections from said resistor `to the second of said windingsfor providing a1- ternating current through the latter in response to said rectified voltages becoming unequal due to a frequency difference between the oscillatory energy frequency and said reference frequency. and means causing said alternating currents to now through said windings in phase quadrature.

MURRAY- G. CROSBY. 

